The present invention relates to a planetary reduction gear having a molded ring gear. More particularly but not exclusively, it relates to a planetary reduction gear having a molded ring gear for use in an internal deceleration-type starter for an automobile or other vehicle, the molded ring gear being improved in that it does not undergo deformation due to sink marks during molding.
An internal deceleration-type starter is a starter in which the starter motor and the reduction gear for the motor are a single unit contained within the same housing. Commonly, the reduction gear for the motor is a planetary gear having an internally-toothed ring gear made of a molded synthetic resin.
FIG. 1 illustrates a molded ring gear for a conventional starter of this type. The ring gear 1 is a single body formed by molding of a synthetic resin and comprises internal teeth 2, an outer rim 3 which surrounds the teeth 2, an inner flange 4 which supports a rotating shaft, and wall 5 which connects the outer rim 3 with the inner flange 4. In the outer periphery of the outer rim 3 are formed two semicircular projections 3a separated by 180 degrees and two semicircular through slots 3b separated from one another by 180 degrees and separated from the projections 3a by a central angle of 30 degrees. The projections 3a fit into grooves in a front frame of a conventional starter and prevent the rotation of the ring gear 1, while bolts pass through the through slots 3b.
This type of ring gear 1 suffers from the problem that during molding, sink marks develop which tend to cause deformation of the ring gear 1. This causes the ring gear 1, which should be perfectly circular, to deform into an oval shape.